The present invention relates to a photographic lens having a large aperture ratio.
As recent years have seen diverse photographic lens users, there has been an increasing demand for higher performance photographic lenses of large aperture ratio. The greatest problem in achieving higher performance in the design of lens systems having a large aperture ratio which are used primarily as photographic lenses is the difficulty in correcting aberrations such as spherical aberration and sagittal coma. If sagittal coma is produced, then the contrast at low frequencies in normal photographing operation is lowered, and a bright point object photographed at night has a light patch flaring away to one side like a tail, resulting in a degraded photographed image. There is known an optical system incorporating an aspherical lens surface for removing spherical aberration and coma to thereby eliminate the foregoing shortcoming. According to the present invention, a gradient refractive index which has been developed and has found use in optical communications in recent years is incorporated in lens elements for higher lens performance.
The presently available techniques for manufacturing lenses with gradient refractive indexes include an ion exchange process, a crystal growing process, a chemical vapor deposition process, and many other processes. With the prior processes, however, the depth of a layer having a gradient refractive index is about 10 mm, and the gradient refractive index varies about 0.06 for practical purposes.
The first photographic lens in which a gradient refractive index is incorporated was designed by Dr. Moore et al. of Rochester University and presented in the international conference on gradient refractive index held in Hawaii, U.S.A. in 1981. According to their design, an ordinary Gaussian photographic lens of six homogeneous elements having spherical surfaces with an F number of 1:1.0 and a focal length of 50 mm can be designed only with two-element lens having a gradient refractive index. Concerning the potential ability of the gradient refractive index, the designed lens has a gradient refractive index varying across an increased depth by .DELTA.N=0.12 in a radial direction normal to the optical axis. The lens manufacturing technique available today however fails to fabricate such a lens.